Thermally self-correcting gain modules and associated systems and methods

ABSTRACT

Aspects of the present invention are directed to the use of optical gain structures that include alternating layers of gain medium and transparent heat conductors in which the gain medium itself functions as a correction optic. The gain medium changes to an optimum or desired shape because of the thermal changes occurring as the materials of the optical gain structure(s) reach a desired optical output condition. At the desired optical output conditions, the gain medium conforms to a desired shape. The desired shape may be, for example, that of an optical surface of a transparent heat conductor. By designing the initial shape of the gain medium such that the physical contact with the transparent heat conductor is maximized at the desired optical output conditions, conductive heat transfer between the gain medium and heat conductor(s) is maximized at the desired optical output condition.

BACKGROUND

Coherent light has many useful qualities and applications. Lasers arethe most common sources of coherent light, and are utilized in manyindustries. For example, lasers are used in industrial manufacturing forvarious processes including cutting, machining and welding of metallicand non-metallic materials. Lasers are also used in thetelecommunications industry to generate and amplify light transmittedover optical fibers, and also in many other applications.

Certain wavelengths of light can be difficult to produce directly fromlasers. This may be due to material properties, e.g., energies of thelight producing electron transitions, or complexities involved withmanaging a particular gain medium, e.g., toxicity of gases or liquidsthat are used. Coherent light may be produced at these otherwiseproblematic wavelengths by using optical parametric devices to shift thewavelength of the output of a laser. Optical parametric devices convertlight of one wavelength to light of another wavelength through theprocess known as three-wave interaction, in which three optical waves orfields are mixed and one or two of the three optical waves may beselectively amplified. Nonlinear crystalline materials are commonlycharacterized as being a particular type, i.e., either Type I or TypeII, according to how the effect of double refraction or birefringenceaffects incident light once it enters the particular crystal. Anonlinear crystal may be referred to as a Type I crystal when anincident or “pump” wave is doubly refracted into signal and idler fieldsor waves that have the same polarization, which is orthogonal to thepump wave. A Type II nonlinear crystal is one producing orthogonallypolarized signal and idler fields or waves from a pump wave.

Efficient heat removal from an active or nonlinear gain medium is a keyissue for any high power operation, and can constrain output powerscaling for a given gain medium. Excess heat, or thermal energy, withina gain medium can decrease the desired gain interaction, whether it belaser or parametric. Deleterious consequences of excessive heat in thegain medium include a reduced population inversion and thermal lensing.Thermal management issues are especially important for solid-statelasers, where, unlike gas and liquid active medium lasers, the activemedium itself cannot be removed from the laser cavity to facilitateproper heat exchange.

Previous attempts have been made to improve thermal management in solidstate lasers to increase power output and/or beam quality. Convectivecooling has been used to remove heat from solid state gain media byhaving a fluid, which may be either a gas or liquid, flow over one ormore surfaces of the gain particular medium. Conductive cooling methodshave been used to remove heat from one or more surfaces of a solid stategain medium, typically by placing a heat sink into contact with one ormore surfaces of the gain medium. Such previous convective andconductive cooling methods can be limited as to the amount of heat thatcan be removed from the gain medium. Because solid state gain media aretypically poor heat conductors and conductors, the rate at which heatcan be removed from the gain medium can be limited by the gain mediumsurface area that is available for heat removal.

A prior art laser in which a given volume of gain medium is separatedinto pieces as a way to increase the surface area available for heattransfer is described in U.S. Pat. No. 6,667,999 to Hasson et al.,commonly owned by the assignee of the present application. FIG. 1 is across section of a prior art laser 100 as described in U.S. Pat. No.6,667,999. The prior art laser 100 includes multiple gain cellsconfigured in a sandwich-like arrangement along an optical axis 101within a resonator formed by first and second mirrors 108, 110. Eachgain cell consists of a disk of laser material 102, such as Nd:YAG,alternating with a disk of an optically transparent heat transfer medium(inline OTH) 104. The inline OTH 104 is described as a diamond disk.Antireflective or index matching coatings 106 are present between thedisks of laser material 102 and the adjacent inline OTH 104. Aperipheral OTH 114 is positioned laterally to the gain cells and incontact with a peripheral surface of the inline OTH. The peripheral OTH114 contacts each gain cell so that heat can be transferred to a heatexchange system (not shown). When the gain cells are optically pumped tocreate laser gain and an optical output 116, waste heat develops in thelaser gain material 102. This heat is conducted parallel to the axialdirection into the inline OTH 104, e.g., diamond disks, where the heatis efficiently conducted radially by the diamond 104 to the peripheralOTH and on to the heat exchange system (not shown). A similar laser andthermal management system is described in H. P Chou, Y. Wang, V. Hasson,“Compact and Efficient DPSS Laser Using Diamond-cooled Technology”,Proc. of SPIE Conf. For HPLA V, Vol. 5448, p 550, Taos, N. Mex., April2004.

Even though the laser 100 of FIG. 1 improves on previous techniques toremove heat from a laser gain medium, it has been observed that whenpump power is increased above a certain value, the specific output for agiven gain medium decreases. FIG. 2 includes FIG. 2A and FIG. 2B, whichdepict, respectively, a gain module 200 of the thermal management system100 of the prior art laser of FIG. 1 at two different operationalconditions. The gain module 200 includes a gain medium 202 placedbetween two inline OTH 204(1)-204(2) along an optical axis 201 of aresonator (not shown). The gain medium 202 is disk shaped and has firstand second optical surfaces 206(1)-(2), first and second lateral sides208(1)-(2), and a width or thickness 214. The two inline OTH 204(1)-(2)are also disk-shaped and each have first and second optical surfaces210(1)-(4), as well as a desired thickness 212. The operationalcondition depicted in FIG. 2A represents a condition in which no opticalpumping is present and consequently no optical output is being producedby the gain medium 202. As shown in FIG. 2A, the first and secondoptical surfaces 206(1)-(2) of the gain medium 202 are each initially incontact with a respective first optical surface 210(2)-(3) of theadjacent OTH 204(1)-(2).

The operational condition depicted in FIG. 2B represents one in whichthe gain medium 202 is producing an optical output exceeding a certainspecific output value along the optical axis 201 due to incident opticalpump energy of a relatively high power density. The heat generatedthrough the optical gain process produces non-uniform thermal expansionof the gain medium 202 as the specific output of the gain medium 202exceeds a certain value. The OTH 204(1)-(2) do not deform significantlybecause diamond has a very high thermal conductivity, i.e., the highestof any known substance at room temperature, as indicated by thickness212′, which is only marginally greater than the initial thickness 212.The OTH 204(1)-(2) can consequently conduct the heat flux with arelatively low thermal gradient in the radial direction relative to theoptical axis 201. Because the laser gain material 202 has a much lowerthermal conductivity than the OTH, larger temperature gradients occur.

With continued reference to FIG. 2B, the temperature gradients withinthe gain medium 202 resulting from the optical gain process producenon-uniform expansion of the gain medium 202 in the direction of theoptical axis 201. This can lead to warping of the optical surfaces206(1)′-(2)′ as indicated by the increased thickness 214′ of the gainmedium 202. The thermal expansion 214′ reaches a maximum value along theoptical axis 201, as shown. Significant internal stresses may develop inthe gain medium 202 if the physical movement of the OTH 204(1)-(2) andgain medium 204 are constrained relative to each other as the thermalexpansion process occurs. The stresses can eventually lead to fractureof the gain material 202. The nature of the non-uniform thermaldistortion is such to form the material of the initially disk-shapedgain medium into a biconvex, lens-like shape, as shown. The thermalexpansion can also cause separation of the gain medium 202 and the OTH204(1)-(2) at their interface, as shown in FIG. 2B. Separation of thegain medium 202 from the diamond OTH leads to reduced beam quality andreduced specific output for the gain medium 202, even as pump energiesincrease beyond a certain value.

What is desirable, therefore, is to provide apparatus, systems, andmethods for thermal management that allow laser and/or parametric gainmedia to operate above present limits of specific output for a givensolid state gain medium.

SUMMARY

Aspects of the present invention are directed to the use of optical gainstructures that include alternating layers of gain medium andtransparent heat conductors in which the gain medium itself functions asa correction optic. The gain medium changes to an optimum or desiredshape because of the thermal changes occurring as the materials of theoptical gain structure(s) reach a desired optical output condition. Atthe desired optical output conditions, the gain medium conforms to adesired shape, which may be, for example, that of an optical surface ofa transparent heat conductor. By designing the initial shape of the gainmedium such that the physical contact with the transparent heatconductor is maximized at the desired optical output conditions,conductive heat transfer between the gain medium and heat conductor(s)is maximized at the desired optical output condition. Consequently,material stresses may be minimized and output beam quality may bemaximized for high specific outputs of the gain medium. Embodiments ofthe present invention are directed to apparatus, systems, and methodsproviding for thermally self-correcting laser and optical parametricsystems.

One embodiment of the present invention includes a thermallyself-correcting optical gain module having a first heat conductorsubstantially transparent to one or more desired wavelengths of light.The first heat conductor has a first coefficient of thermal expansion, afirst coefficient or thermal conductivity, and first and second opticalsurfaces. A first gain medium operable to produce light at one or moredesired wavelength and having first and second optical surfaces isdisposed adjacent to the first heat conductor. The first gain medium hasa second coefficient of thermal expansion greater than the firstcoefficient of thermal expansion. The first gain medium has a secondcoefficient of thermal conductivity lesser than the first coefficient ofthermal conductivity. The first gain medium is operable to receive pumpenergy from a means for pumping. The first optical surface of the firstgain medium has a predetermined shape at a first optical outputcondition. The first optical surface of the first gain medium issubstantially dissimilar to the first optical surface of the first heatconductor at the first optical output condition. The first opticalsurface of the first gain medium substantially conforms to and contactsthe first optical surface of the first heat conductor at a secondoptical output condition. Heat conduction can occur from the first gainmedium to the first heat conductor through an interface formed by thefirst optical surface of the first heat conductor and the first opticalsurface of the first gain medium at the second pumping condition.

The gain medium can be a laser gain material. The gain medium mayinclude YAG, and may be Nd:YAG. The gain medium can be include anonlinear gain medium. The nonlinear gain medium may include KTA. Thefirst optical surface of the first heat conductor may be substantiallycircular. The first gain medium may be shaped as a disk. The firstoptical surface of the first gain medium may be concave toward the firstoptical surface of the first heat conductor. The first optical surfaceof the first gain medium may have a substantially Gaussian orHermite-Gaussian profile relative to the optical axis. The first opticalsurface may have a Gaussian profile relative to the optical axis. Thefirst transparent heat conductor may be shaped as a disk that isconcentric, or axisymmetric, with the optical axis.

The gain module may include a second gain medium disposed on the opticalaxis and adjacent to the second optical surface of the first heatconductor. The second gain medium when present is operable to producelight at one or more desired wavelengths and has first and secondoptical surfaces. The second gain medium has the second coefficient ofthermal expansion, and is operable to receive pump energy from a meansfor pumping. The first optical surface of the second gain medium has apredetermined shape at a first optical output condition. Thepredetermined shape is substantially dissimilar to the second opticalsurface of the first heat conductor at the first optical outputcondition. The first optical surface of the second gain mediumsubstantially conforms to and contacts the second optical surface of thefirst heat conductor at a second optical output condition. Heatconduction can occur from the second gain medium to the first heatconductor through an interface formed by the second optical surface ofthe first heat conductor and the first optical surface of the secondgain medium at the second optical output condition. The gain module mayinclude a second heat conductor disposed adjacent to the first gainmedium on the optical axis. The second heat conductor is substantiallytransparent to one or more desired wavelengths of light. The second heatconductor has the first coefficient of thermal expansion, the secondcoefficient of thermal conductivity, and first and second opticalsurfaces. The first optical surface of the second heat conductor isdisposed adjacent to the second optical surface of the first gainmedium.

The first transparent heat conductor may include diamond, which may besingle crystal diamond. The first transparent heat conductor may includesapphire. The gain module may include one or more anti-reflective orindex matching coatings disposed between the first optical surface ofthe first gain medium and the first optical surface of the first heatconductor. The one or more anti-reflective or index matching coatingshave refractive indexes between a refractive index of the first gainmedium and the first heat conductor. Suitable AR and/or index matchingcoatings may be deposited by know deposition techniques, e.g.,sputtering, and may be of desired thickness, e.g., multiples ofquarter-wavelengths of light produced by an associated gain medium. Suchcoatings may include one or more layers of aluminum oxide (alumina, orAl₂O₃), tantalum pentoxide (Ta₂O₅), magnesium fluoride (MgFl₂), silicondioxide (SiO₂), titanium oxide (TiO), or combinations thereof, incertain embodiments.

A further embodiment includes a thermally self-correcting opticalresonator system. The system has a first mirror having a firstreflecting surface with a first reflectivity at a desired wavelength anda second mirror having a first reflecting surface with a secondreflectivity at the desired wavelength. The second reflectivity isdifferent from the first reflectivity. The first reflecting surface ofthe second mirror is configured and arranged to reflect light along anoptical axis to the first reflecting surface of the first mirror. Thefirst and second mirrors are operable as a resonator. A first heatconductor disposed on the optical axis. The first heat conductor issubstantially transparent to one or more desired wavelengths of light.The first heat conductor has a first coefficient of thermal expansion, afirst coefficient of thermal conductivity, and first and second opticalsurfaces. The system includes a first gain medium operable to producelight at one or more desired wavelengths and having first and secondoptical surfaces. The first gain medium is disposed adjacent to thefirst heat conductor on the optical axis. The first gain medium has asecond coefficient of thermal expansion greater than said firstcoefficient of thermal expansion. The first gain medium has a secondcoefficient of thermal conductivity lesser than the first coefficient orthermal conductivity. The first gain medium is operable to receive pumpenergy from a means for pumping. The first optical surface of the firstgain medium has a predetermined shape at a first optical outputcondition corresponding to no pump energy. The predetermined shape issubstantially dissimilar to the first optical surface of the first heatconductor at the first optical output condition. The first opticalsurface of the first gain medium substantially conforms to and contactsthe first optical surface of the first heat conductor at a secondoptical output condition. Heat conduction can occur from the first gainmedium to the first heat conductor through an interface formed by thefirst optical surface of the first heat conductor and the first opticalsurface of the first gain medium at the second optical output condition.

The thermally self-correcting optical resonator system may include ameans for pumping operable to produce pump energy. The means for pumpingis configured and arranged to transmit pump energy to the first gainmedium. The resonator of the system can be a stable resonator or anunstable resonator. The means for pumping may include one or more diodebars. The first heat conductor may include diamond, which can be singlecrystal diamond. The means for pumping may be configured and arrangedsuch that pump light is incident on the first transparent heat conductorat the Brewster angle between the first heat conductor and anintermediary optical medium located between the means for pumping andthe first heat conductor. The intermediary optical medium can be air.The intermediary optical medium can be water. The system may include abeam combiner disposed on the optical path in the resonator to receivethe pump energy from the means for pumping. When present, the beamcombiner is configured and arranged to transmit the pump energy alongthe optical path. The beam combiner may include one or more Brewsterprisms. The first mirror can be a graded reflectivity mirror. The gradedreflectivity mirror may include one or more optical coating with gradedreflectivity profiles disposed on the first optical surface of the firstmirror, and the coatings may support a single transverse mode along theoptical axis. The graded reflectivity mirror may include a Gaussianreflectivity profile. One or more dichroic coatings may be disposed onthe first and second mirrors within the resonator. The dichroic coatingsmay transmit the pump wavelength and have a desired reflectivity for awavelength generated by the first gain medium.

A further embodiment includes a method of producing light using aself-correcting gain medium in conjunction with a transparent heatconductor. A first gain medium having first and second optical surfaces,with the first optical surface having a thermally self-correcting shape,may be placed adjacent a heat conductor having first and second opticalsurfaces and being transparent at one or more desired opticalwavelengths. The thermally self-correcting shape is dissimilar to theshape of the first optical surface of the heat conductor at an initiallevel of optical output of the first gain medium. Pump energy is addedto the first gain medium. Thermal energy is added to the first gainmedium by the associated gain process. The first gain medium produces adesired optical wavelength. The first optical surface of the first gainmedium conforms to the first optical surface of the heat conductor at adesired level of optical output of the gain medium. Heat is removedaxially from the first gain medium through the first optical surface ofthe heat conductor. Heat is removed radially from the heat conductor.The step of adding thermal energy to the first gain medium can beselected from the group consisting of amplifying fluorescence,amplifying a signal wave, amplifying an idler wave, and amplifying apump wave.

BRIEF DESCRIPTION OF DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings. The drawingsare not necessarily to scale, emphasis instead being placed onillustration of principles of the invention. The drawings include thefollowing figures:

FIG. 1 is a side cross-sectional view of a prior art laser.

FIG. 2 includes FIG. 2A and FIG. 2B, which depict side cross-sectionalviews of a gain cell from the laser of FIG. 1 at two differenttemperatures.

FIG. 3 is a side cross-sectional view of an illustrative gain moduleaccording to the present invention.

FIG. 4 includes FIG. 4A and FIG. 4B, which depict side cross-sectionalviews of an alternate illustrative embodiment of a gain module accordingto the present invention.

FIG. 5 is a side cross-sectional view of an alternate illustrativeembodiment according to the present invention.

FIG. 6 is a side cross-sectional view of an alternate illustrativeembodiment according to the present invention in which a transparentheat conductor is positioned between two portions of gain medium.

FIG. 7 is a side cross-sectional view of an illustrative embodiment of aresonator and gain system according to the present invention.

FIG. 8 is a side cross-sectional view of an illustrative resonatorsystem having an unstable resonator and pump arrays in a Brewster angleconfiguration.

FIG. 9 is a side cross-sectional view of an alternate illustrativeembodiment of a resonator system having an unstable resonatorconfiguration.

FIG. 10 depicts steps in an illustrative method of producing light usinga self-correcting gain medium according to the present invention.

DETAILED DESCRIPTION

The present invention may be understood by the following detaileddescription, which should be read in conjunction with the attacheddrawings. The following detailed description of certain embodiments isby way of example only and is not meant to limit the scope of thepresent invention.

Aspects of the present invention are directed to the use of optical gainstructures that include alternating layers of gain medium andtransparent heat conductors in which the gain medium itself functions asa correction optic. The gain medium changes to an optimum or desiredshape because of the thermal changes occurring as the materials of theoptical gain structure(s) respond to desired pumping conditions. At thedesired pumping conditions, the gain medium conforms to a desired shape,for example, an optical surface of an adjacent transparent heatconductor. By designing the desired shape of the gain medium such thatthe thermal contact with the transparent heat conductor is maximized atdesired optical pumping and/or optical output conditions, materialstresses may be minimized in the gain medium and output beam quality maybe maximized for high specific outputs. Embodiments of the presentinvention are directed to apparatus, systems, and methods providing forthermally self-correcting laser and optical parametric systems.

FIG. 3 is a side cross-sectional view of a gain module 300 according tothe present invention. The gain module 300 includes a suitably formedgain medium 302 and a transparent heat conductor 304 adjacent to oneanother on an optical axis 301. The gain module 300 is shown in acondition representing a condition less than a desired specific outputfor the gain medium 302. The gain medium 302 has first and secondoptical surfaces 306(1)-(2) along the optical axis 301. The first andsecond optical surfaces 306(1)-(2) have respective predetermined shapes,e.g., profiles 308(1)-(2), described in greater detail hereinafter. Thetransparent heat conductor 304 has first and second optical surfaces310(1)-(2). One optical surface 310(2) of the gain medium 302 isconfigured and arranged to receive light from the gain medium 302. Thegain medium 302 may be optically pumped by a suitable means for pumping(not shown) and is operable to produce light at a desired wavelengththrough an optical gain process.

The profile of each optical surface of the gain medium is such that theoptical surface conforms to any input surface of an adjacent transparentheat conductor at desired pumping conditions. The optical surfaceprofiles 308(1)-(2) may be derived from detailedthermo-mechanical-optical calculations, described in more detail below.The result is a desired shape for the gain medium optical surfaces atdesired specific output conditions at a design optical pumping load(kW/cm³). The desired shape of the optical surfaces at the desiredoutput conditions may be such as to maximize (i) surface contact withthe adjacent optical face(s) of a transparent heat conductor 304, and(ii) maximize beam quality for an output beam produced by the gainmedium 302. In some applications, the shape of gain media opticalsurfaces may have profiles 308(1)-(2) that are Gaussian orHermite-Gaussian, to accommodate output beam intensity of the TEM₀₀ modeof an associated stable resonator or a deterministic profile from aunstable resonator, with or without a graded reflectivity outputcoupler, or mirror. In certain embodiments, the profiles 308(1)-(2) maybe different from one another. For example, the degrees of concavity ofeach optical surface 306(1)-(2) may be different as illustrated bydissimilar respective distances 312 and 314 on the optical axis 301.These differences may depend on thermal conductivity values adjacent tothe respective optical surfaces 306(1)-(2), such as when a transparentheat conductor is adjacent to only one side of the gain medium 302.

The materials used for the gain media 302 may be active or “laser” gainmedia or may be optical parametric gain media. Suitable types of lasergain media may include solid state media, including solid state gainmedia that have high power and thermal loading material properties. Incertain embodiments, the gain medium 302 may include nonlinear, orbirefringent, optical crystalline materials as parametric gain media. Inpreferred embodiments, the gain module 300 includes thin disks of Nd:YAGlaser gain material alternated between thin disks of transparent heatconductors 304 made of single crystal synthetic diamond. Diamond's heatconductivity is over 2000 W/m° K. and at room temperature its value isthe highest of any known substance. Other materials may be used, fornon-limiting example, sapphire, which has a heat conductivity of 40 W/m°K.

With continued reference to FIG. 3, one or more suitable antireflection(AR) or index matching coatings 316 may be present between the gainmedium 302 and transparent heat conductor 304 on the optical axis 301.Such coatings 316 may be disposed on an optical surface 310 of thetransparent heat conductor 304, as shown in FIG. 3, or an opticalsurface, e.g., 306(1) of the adjacent gain medium 302. In certainembodiments, suitable AR and/or index matching coatings may include oneor more layers of aluminum oxide (alumina, or Al₂O₃), tantalum pentoxide(Ta₂O₅), and/or magnesium fluoride (MgFl₂) of desired thickness, e.g.,quarter-wavelengths of light produced by an associated gain medium.Suitable AR and/or index matching coatings may include silicon dioxide(SiO₂) and/or titanium oxide (TiO), in certain embodiments.

In certain applications, the profiles of the gain medium 302 may bederived by performing mathematical analyses that involve a numericalprocedure to solve coupled heat transfer and electromagnetic wavepropagation equations at the interface of the gain medium-transparentheat conductor interface, e.g., diamond-YAG interface, under variouslaser pump powers. Self-consistent solutions for temperature profilesfor each contact surface may be obtained by an iterative procedure inwhich solutions converge, for non-limiting example, by applying theNewton-Raphson method. Other suitable iterative methods may be used,including, but not limited to, the Chebyshev iteration method and othermethods that use orthogonal polynomials. The solutions for temperaturedistributions in the various substances involved, e.g., diamond of theheat conductor 304, index matched coatings 316, a gain medium 302including YAG, etc., can in turn provide refractive index profileswithin each material, respectively. Solving the EM wave equation forlight propagation in an inhomogeneous medium, whose refractive indexprofiles have been determined previously, leads to the solutions forreflective as well as transmitted waves at the interface between thegain medium and transparent heat conductor. These calculations may becarried out as a function of pump spatial power density conditions. Heattransfer characteristics and optical performance of any included ARcoatings can be taken into account for such calculations.

The relevant physics may be modeled using suitable coupled-physicsfinite element modeling and analysis software. For example,ANSYS-MULTIPHYSICS or FEMLAB modeling software may be used for thethermo-mechanical and electromagnetic (EM) wave propagation calculationsto determine a suitable initial shape of the optical surfaces of a gainmedium under conditions of no pumping. Such software can be used to bemodel the coupled physics of a gain medium with a given initial shapeunder desired optical conditions (e.g., pumping, type of optical gainprocess, gain coefficient) subject to thermal boundary conditions thatallow a desired specific output for the gain medium to be achieved.

In certain applications, the initial shape of the gain medium opticalsurfaces for non-pumped condition can be derived by using suitablesoftware to take the final shape of the gain medium 302 at the desiredoperational conditions, subject that final shape to the thermalexpansion occurring at the desired operational conditions, and thenreverse the resulting shape. For example, assuming that at its desiredoperational condition the gain medium 302 is to conform to a disk-shapeddiamond heat conductor 304, ANSYS-MULTIPHYSICS or FEMLAB modelingsoftware may be used to model the thermal expansion that a disk of thesame gain material undergoes at the desired operational conditions,e.g., desired specific output subject to the thermal boundary conditionspresented by the heat conductors and EM wave conditions within anassociated resonator. For a disk of gain material, such thermal boundaryconditions may include (i) an insulated circumferential surface, and(ii) a heat flux, e.g., constant or linear, per unit area over the firstand second optical surfaces to model the high heat conductivity ofassociated transparent heat conductors. By reversing, or flipping, theshapes of the thermally deformed optical surfaces of the modeled disk ofgain medium relative to the optical axis, and using this reversed shapefor the predetermined shape of the actual gain medium, the opticalsurfaces of the actual gain medium will be optimized to conform to thedesired shape, e.g., a flat disk, at the desired operational conditions.The preceding is one way to determine the thermally-self-correctingshape of the gain medium, and other ways may be used. Other suitablesoftware may be also be used, e.g., ZEMAX software for the EM wavepropagation. ANSYS-MULTIPHYSICS is a trademark for engineering analysissoftware by ANSYS, Inc., of Southpointe, 275 Technology Drive,Canonsburg, Pa. 15317. FEMLAB is a registered trademark for engineeringanalysis software by COMSOL, Inc., 1 New England Executive Park Suite350 Burlington, Mass. 01803. ZEMAX is a registered trademark for opticaldesign software by ZEMAX Development Corporation, 4901 Morena Blvd.Suite 207, San Diego, Calif., 92117-7320 USA.

The desired shape of the optical surfaces 306(1)-(2) and associatedprofiles 308(1)-308(2) can be calculated, as expressed above. One canalso improve the calculated result by obtaining measurements of theimpact on the wave front of an optical beam passed through the gainmodule when being pumped 300. By doing such measurements at a sequenceof pump power conditions, e.g., up to a certain amount of thermalexpansion of the gain medium on the optical axis, the model of theprofiles 308(1)-308(2) may be validated and anchored. In theinterpretation of the data, one may distinguish between opticaldistortions due to thermal expansion and thermally induced changes inthe refractive index of the gain material.

FIG. 4 includes FIG. 4A and FIG. 4B, which depict respective sidecross-sectional views of a gain module 400 according to the presentinvention. The gain module 400 includes alternating gain media402(1)-(2) that alternate with optically transparent heat conductors404(1)-(3) in a stacked, or sandwich-like, configuration, as shown. Thegain media 402(1)-(2) and heat conductors 404(1)-(3) are arranged on anoptical axis 401, and the optical surfaces 410(1) and 410(6) arearranged to pass light along the optical axis 401. The optical axis maybe associated with an optical resonator of suitable design (not shown).Each of the gain media 402(1)-(2) includes respective first and secondoptical surfaces 406(1)-(4) that are adjacent to corresponding opticalsurfaces 410(2)-(5) of the transparent heat conductors 404(1)-(3). Anysuitable solid state gain material may be used for the gain media402(1)-(2). For non-limiting example, the gain material may be anysuitable solid state laser gain medium or suitable nonlinear gainmedium.

FIG. 4A shows the gain module 400 in a lower-temperature state, e.g.,room temperature, in which the first and second optical surfaces of thegain media are dissimilar to and substantially not in contact withtouching the heat conductors 404(1)-(3). The optical surfaces of thegain media 402(1)-(2) are pre-formed into a desired shape, e.g.,concave, to accommodate thermal expansion of the gain material atdesired operational conditions. Index-matching layers 416(1)-(4) mayoptionally be present between adjacent gain media 402(1)-(2) and heatconductors 404(1)-(3) to facilitate optical power flow through the gainmodule 400 by minimizing Fresnel reflection losses. The refractive indexof the index-matching material 416(1)-(4) may be selected as desired. Incertain embodiments, refractive index of the index-matching material416(1)-(4) may be equal or about equal to the geometric mean of therefractive indexes of the gain media 402(1)-(2) and the heat conductors404(1)-(3).

FIG. 4B shows the gain module 400 of FIG. 4A in an operational conditionthan of the condition shown in FIG. 4A. The operational condition maycorrespond to a desired pumping load and/or specific output for the gainmedia 402(1)-(2). After undergoing thermal expansion, due to thedesigned pumping and gain process, the layers of gain media 402(1)-(2)conform to the proximal surface(s) of the transparent heat conductors404(1)-(3). Conductive eat transfer occurs from the gain media402(1)-(2) to the heat conductors 404(1)-(3), in the direction of theoptical axis 401. The heat is then rapidly conducted radially outward,in a direction perpendicular to the optical axis 401, through thetransparent heat conductors 404(1)-(3) to ancillary thermal managementsystem components, e.g., a cooling fluid circulating at thecircumference of the gain module 400. This geometry effectively removesthe heat from the gain medium 402(1)-(2) in a manner that permits theattainment of high power output with high beam quality.

With continued reference to FIG. 4A, when starting from a condition ofno pumping, the transition to a desired final shape of the gain media,i.e., one that conforms to the optical surfaces of adjacent heatconductors, is made faster by the initial poor physical contact betweenthe gain media 402(1)-(2) and the heat conductors 404(1)-(3). This poorinitial physical contact, which is because of the gap separating thegain material 402(1)-(2) and the adjacent heat conductors 404(1)-(3),provides little opportunity for heat conduction from the gain media402(1)-(2) to the heat conductors 404(1)-(3). The initial opticalperformance may be relatively poor at initial pumping conditions becauseof the gap that is present, which will lead to a larger fraction of pumppower ending up as waste heat. This in turn may accelerate thetransition to a desired final shape of the gain media 402 and level ofoptical performance. The number of transparent heat conductors may beselected as desired. For example, the number of heat conductors 404 maybe selected based on the heat removal needs for a given volume of gainmaterial, pumping loads, and desired specific output.

FIG. 5 shows a cross section of a gain module 500 according to analternate embodiment of the present invention in which a manifold 506 isused in conjunction with a fluid 508 for heat removal. Gain media502(1)-(2) with desired coupling surface profiles alternate withtransparent heat conductors 504(1)-(3), as shown, similar to theembodiment of FIG. 4. The manifold 506 surrounds the lateral surfaces ofthe gain module 500. Windows, or recesses, 510(1)-(2) are present in themanifold 506 to allow optical pumping of the gain media 502(1)-(2) alongthe optical axis 501 and also to allow the optical output to exit thegain module 500. It will be understood that while one direction of fluidflow 514 is show, other configurations of the manifold 506 and fluidflow are possible.

FIG. 6 shows another embodiment 600 of a gain module according thepresent invention. In this embodiment, two gain media 602(1)-(2) areshown positioned on either side of a transparent heat conductor 604along an optical axis 601. Antireflection coatings 616(1)-(2) mayoptionally be present between the gain media 602(1)-(2) and thetransparent heat conductor 604. Due to the different heat conductionproperties on the optical surfaces 606(1)-(4) of the gain media602(1)-(2), each optical surface of an individual gain mediumexperiences a different thermal expansion at a desired operationalcondition. The initial shapes, e.g., profiles 607(1)-(2) relative to theoptical axis 601, of the respective optical surfaces 606(1)-(4) of thegain media are accordingly different in the embodiment shown. Profiles607(1)-(2) that are not adjacent to a transparent heat conductor 604 mayhave a higher degree of concavity 614 than profiles 608(1)-(2) that areadjacent to the heat conductor 604 to accommodate increased thermalexpansion.

FIG. 7 is a side cross-sectional view of an embodiment of a resonatorsystem 700 according to the present invention. The system 700 includes again module 711 with alternating layers of gain media 702(1)-(2) andtransparent heat conductors 704(1)-(3), similar to the embodiment ofFIG. 4. The gain module 711 is disposed between first and second mirrors706, 708, forming a resonator 703 that is operable to produce an output719. The gain module 711 is pumped by pump energy 714 from a means forpumping. The means for pumping may be any suitable light source 712,such as a diode array, a flash lamp, a suitable laser, etc. Collimatingand/or imaging optics, such as lenses 716 and 718, may be present on theoptical axis 701 to direct the pump energy to the gain media 702(1)-(2).Even though the gain layers 702(1)-(2) are shown in FIG. 7 as nottouching the transparent heat conductor 704(1)-(3), it will beunderstood that once certain operation conditions are met, the gainmedia 702(1)-(2) will contact the heat conductors 704(1)-(3), e.g.,similar to the configuration shown in FIG. 3B. Dichroic coatings710(1)-(2) may be present on the first and second mirrors 706, 708 toallow unwanted pump energy to exit the resonator 703 while maintainingdesired reflectivities for the amplified light energy. The presence ofdichroic coatings 710(1)-(2) can consequently decrease the potential formaterial damage to the mirrors 706, 708.

FIG. 8 is a side cross-sectional view of a resonator and thermalmanagement system 800 having gain modules 811(1)-(2) and first andsecond mirrors 806, 808. The first and second mirrors 806, 808 areconfigured to operate as resonator 803 having an optical axis 801. Thegain modules 811(1)-(2) are located on the optical axis 801. Similar topreviously described embodiments, the gain modules 811(1)-(2) includegain media 802(1)-(2) placed between transparent heat conductors804(1)-(4). The system 800 includes pump arrays 810(1)-(4) configured ina Brewster angle configuration for the transparent heat conductormaterial, e.g., diamond, and an intermediary optical medium, e.g., air.The Brewster angle 818 for a given pair of optical materials, or media,is equal to the arctangent of the refractive index of the first opticalmaterial, or medium, divided by the refractive index of the secondoptical material, or medium. At the Brewster angle, one polarization oflight is completely transmitted through the boundary of the two opticalmaterials or media, with no power losses for that polarization due toFresnel reflection, i.e., no losses occur due to the difference inrefractive index. In the Brewster angle configuration the pump arraysdirect optical pump energy 814(1)-(4) to the gain cells 811(1)-(2) at anincidence angle equal to the Brewster angle 818 of the heat conductorsand an intermediary optical medium. Such Brewster angle configurationsmay be desirable in certain embodiments because no pump energy isdirected along the optical axis 801, doing away with any need fordichroic coatings (such as are depicted in FIG. 7) on the end mirrors806, 808. Collimating and/or imaging optics 812(1)-(4) may optionally bepresent to facilitate directing the pump energy 814(1)-(4) to the gainmodules 811(1)-(2).

With continuing reference to FIG. 8, resonator 803 is depicted as anunstable resonator. So-called “unstable” resonators are referenced assuch because of the instability of the resonant modes within suchresonators. A stable resonator is one in which a light ray inside theresonator will remain close to the optical axis upon multiplereflections between the end mirrors of the resonator and thus remainconfined within the resonator; the modes of a such a resonator arestable. An unstable resonator is one in which a light ray inside theresonator does not remain confined within the resonator after multiplereflections between the end mirrors. For unstable resonators, thetypical optical mode profile, which is unable to be expressed explicitlyby Gaussian/Gaussian-Hermite polynomials, depends upon the geometry andmirror configuration of the resonator cavity. Optical resonators can beclassified according to a condition known as “stability”. Unstableresonators are characterized by an inherent telescopic magnification ormagnification factor, M. The cross-sectional size of optical modesformed in unstable resonators continually grows by a factor, e.g., M²,on each roundtrip through the resonator. Because the optical modesexpand in an unstable resonator, more of a given volume of gain materialcan be utilized for the associated optical gain process. Unstableresonators may consequently be well suited for high power lasers and/orparametric devices.

The stability condition of a simple two mirrors standing wave resonatorcan be determined by reference to the well known stability criterion,0≦[1+(d/R₁)][1+(d/R₂)]≦1; where d is the length of the optical axiswithin the resonator, and R₁ and R₂ each represent a radius of curvaturefor the respective resonator mirrors. The bracketed values in thestability criterion are often referred to as the resonator stabilityparameters. The stability criterion is often represented graphically asa hyperbola when the respective stability parameters are selected thevalues for the coordinate axes. Resonator geometries corresponding topoints within the region bounded by the hyperbola and the coordinateaxes are stable resonator geometries while those outside of the boundedregion are unstable. Where the product of stability parameters is apositive value, an unstable resonator is said to be a “positive-branch”unstable resonator. Similarly, where the product of stability parametersis a negative value, an unstable resonator is said to be a“negative-branch” unstable resonator.

For embodiments of the present invention, any suitable unstableresonator design may be used, e.g., a positive-branch or negative-branchunstable resonator. The unstable resonator 808 may be a confocalunstable resonator in certain embodiments. Suitable confocal unstableresonator designs include confocal-planar and confocal-convex types. Incertain embodiments, the mirrors used in the unstable resonator asoutput couplers, e.g., mirror 808, may include one or more opticalcoatings having a graded reflectivity profile, in which case the mirrorsmay be referred to as graded-reflectivity mirrors (GRM). A GRM may allowsingle transverse mode (STM) oscillation at a beam diametersubstantially greater than the typical TEM₀₀ mode size to avoid opticaldamage to the components in the resonator. Such graded reflectivityprofiles for a GRM may include, by way of non-limiting example, asubstantially Gaussian or super-Gaussian profile, to improve thecharacteristics of the output beam. Suitable GRM may include a highreflectivity central region known as a dot reflector. In certainapplications, a GRM may include an apodizing, or smoothing, element tolimit the output beam diameter.

FIG. 9 is a side cross-sectional view of an alternate embodiment of aresonator and thermal management system 900 having an unstable resonatorconfiguration similar to FIG. 8. The system 900 includes gain modules911(1)-(2) and beam combiners 918(1)-(2) located on an optical axis 901within an unstable resonator 903 formed by first and second mirrors 906,908. The gain modules 911(1)-(2) include gain media 902(1)-(2) disposedbetween transparent heat conductors 904(1)-(4). The beam combiners918(1)-(2) are configured and arranged to direct optical pump energy914(1)-(4) from pump light sources 910(1)-(4) to the gain modules911(1)-(2). The gain modules 911(1)-(2) are configured and arrangedwithin the resonator 903 such that light received from the beamcombiners 918(1)-(2) is incident at the Brewster angle 920 for the heatconductors and an intermediary optical medium, e.g., air.

The beam combiners 918(1)-(2) of FIG. 9 may be Brewster beam combiners,e.g., pairs of Brewster prisms in certain applications. The pump lightsources 910(1)-(4) are not necessarily orthogonal to the optical axis901. Collimating and/or focusing optics 912(1)-(4) may be present asshown to facilitate pumping of the gain modules 911(1)-(2). Dichroiccoatings 909 may be present on the first and second mirrors 906, 908 toallow the mirrors to transmit unwanted pump energy while maintainingdesired reflectivities for the amplified light energy, thus decreasingmaterial damage potential for the mirrors 906, 908, similar to theembodiment of FIG. 7.

FIG. 9 also includes an optical shutter 922 placed on the optical axiswithin the resonator 903. The optical shutter allows the system 900 tooperate in quasi-CW or pulsed manner. Any suitable techniques or meansmay be used to produce quasi-CW or pulsed operation. For example,Q-switching or cavity dumping may be used to spoil the resonator cavityand prevent oscillation except for a very brief time during anoperational cycle. In certain embodiments, one or more accousto-opticalmodulators may be used for Q-switching. There are various other suitablemethods for Q-switching. For example, one of the resonator mirrors,e.g., 906, may be rotated at a desired rate about an axis perpendicularto the optical axis of the laser cavity. An electro-optic shutter, suchas a Pockels cell, may be used as the optical shutter 922. A saturableabsorber, typically a certain type of dye that bleaches out or becomestransparent when strongly irradiated so that the upper levels aresaturated and thus no more absorption can occur, may be used forQ-switching in certain embodiments.

FIG. 10 shows steps in a method 1000 of producing light using aself-correcting gain medium in conjunction with a transparent heatconductor. An optical gain medium having first and second opticalsurfaces, where the first optical surface has a thermallyself-correcting shape, may be placed 1002 adjacent a heat conductorhaving first and second optical surfaces. The heat conductor istransparent at one or more desired optical wavelengths. The thermallyself-correcting shape is dissimilar to the shape of the first opticalsurface of the heat conductor at an initial level of optical output ofthe gain medium. Pump energy is added 1004 to the gain medium. Thermalenergy is added 1006 to the gain medium by an optical gain process. Adesired optical wavelength is produced 1008 by the gain medium at adesired specific output. The first optical surface of the gain mediumconforms 1010 to the first optical surface of the heat conductor at adesired optical output condition of the gain medium, e.g., a specificoutput level. Heat is removed 1012 axially from the gain medium throughthe first optical surface of the heat conductor. Heat is then removedradially from the heat conductor. The step of adding thermal energy 1004to the gain medium may occur during a gain process including laseramplification of fluorescence or parametric amplification of a signal,idler or pump wave.

Accordingly, embodiments of the present invention may offer advantagesover the prior art. Modeling calculations and preliminary research datahave shown that embodiments may be well suited to repetitively pulsedand CW lasers with high specific output powers, e.g., greater than orequal to 2 kW/cc from active materials such as Nd:YAG. Embodiments maybe used to efficiently remove heat from solid state gain media,including laser and parametric gain media, to allow high power operationof the gain media. Anti-reflection coatings may be used to minimizeoptical transmission loss in order to maximize the efficiency of theassociated gain medium/media. Embodiments may also provide for improvedbeam quality at desired power outputs without requiring the use of fixedoptics. Embodiments offer design flexibility and the geometry of anyresonator used can vary greatly within the scope of the presentinvention. For example, both stable and unstable resonator geometriesand architectures can be used. Resonator according to the presentinvention may be used in master oscillator and/or power amplifierarchitectures. Embodiments may also include rings resonatorconfigurations.

By non-limiting example, embodiments of the present invention may beused in material modification and/or fabrication applications. Certainembodiments may be used in highly-miniaturized, high brightness,fieldable systems for ground, airborne, and space-based applications.Embodiments may be applied to or used in a wide variety of CW and pulsedsolid-state laser architectures and may be used for laser illuminatorsand beacons, ladars/lidars, and countermeasure transmitters as well asdirected energy weapons systems. Certain embodiments may be used forparametric devices, including optical parametric generators and opticalparametric oscillators.

Examples of suitable solid state laser gain media include, but are notlimited to, chromium-doped colquiriite crystals including lithiumstrontium aluminum fluoride (LiSAF), lithium strontium gallium fluoride(LiSGaF), and lithium calcium aluminum fluoride (LiCAF) and cerium-dopedlithium strontium aluminum fluoride (Ce:LiSAF). Yttrium aluminum garnet(YAG) or yttrium lithium fluoride (YLF) doped with trivalent laseractivator ions from both the Rare Earth and Transition Metal groups,e.g., neodymium (Nd), chromium (Cr), erbium (Er), holmium (Ho), thullium(Tm), and ytterbium (Yb), may be used as gain media in certain otherembodiments.

In certain embodiments, suitable nonlinear crystals used as a parametricgain medium may include, but are not limited to, crystals of ammoniumdiphosphate (NH₄H₂PO₄ or “ADP”), beta (β) barium borate (BBO), galliumselenide (GaSe), barium lithium niobate (Ba₂LiNb₅O₁₅), cadmium galliumsulfide (CdGa₂S₄), cadmium selenide (CdSe), cadmium germanium diarsenide(CdGeAs₂), lithium niobate (LiNbO₃), lithium tantalate (LiTaO₃), lithiumtriborate (LiB₃O₅ or “LBO”), potassium diphosphate (KH₂PO₄ or “KDP”),potassium titanyl phosphate (KTiOPO₄ or “KTP”), and potassium titanylarsenate (KTiOAsO₄ or “KTA”). Such media may be of either Type I or TypeII nonlinear crystals.

Although certain embodiments of the present invention have beendescribed, other versions are possible. For example, while certainembodiments have been described as including transparent heat conductorsmade from diamond, other materials may be used for the transparent heatconductor. For example, sapphire may be used in certain embodiments.Other suitable materials may be used. Furthermore, while shapes of heatconductors described above have been described generally as disks ofsingle crystal diamond, other shapes and configurations may be employed.For example, suitable transparent heat conductors may be configured as amosaic of multiple pieces of transparent heat conductors that fittogether in the desired shape for a desired beam size. For example,multiple disks of single crystal diamond may be arranged in a circularmosaic to accommodate a desired beam size. For further example, whilethe optical surfaces of gain media have been described as conforming toa planar surface of a heat conductor at desired output conditions, theoptical surfaces of the gain media may be designed to conform to anydesired shape at the desired output conditions. Additionally, whilemeans for pumping have been described as supplying optical energy to again medium, suitable means for pumping may supply other types of pumpenergy in certain embodiments. For non-limiting example, electricalpumping may be used for certain applications.

While the present invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. All the features disclosedin this specification, including any accompanying claims, abstract, anddrawings, may be replaced by alternative features serving the same,equivalent or similar purpose, unless expressly stated otherwise.

1. A thermally self-correcting optical resonator system comprising: afirst mirror having a first reflecting surface with a first reflectivityat a desired wavelength; a second mirror having a first reflectingsurface with a second reflectivity at said desired wavelength, saidsecond reflectivity being different from said first reflectivity, saidfirst reflecting surface of said second mirror configured and arrangedto reflect light along an optical axis to said first reflecting surfaceof said first mirror, wherein said first and second mirrors are operableas a resonator; a first heat conductor disposed on said optical axis,said first heat conductor being substantially transparent to one or moredesired wavelengths of light, said first heat conductor having a firstcoefficient of thermal expansion, a first coefficient of thermalconductivity, and first and second optical surfaces; and a first gainmedium operable to produce light at one or more desired wavelengths andhaving first and second optical surfaces, said first gain mediumdisposed adjacent to said first heat conductor on said optical axis,said first gain medium having a second coefficient of thermal expansiongreater than said first coefficient of thermal expansion, said firstgain medium having a second coefficient of thermal conductivity lesserthan said first coefficient or thermal conductivity, wherein said firstgain medium is operable to receive pump energy from a means for pumping,wherein said first optical surface of said first gain medium has apredetermined shape at a first optical output condition corresponding tono pump energy, wherein said first optical surface of said first gainmedium is substantially dissimilar to said first optical surface of saidfirst heat conductor at said first optical output condition, and whereinsaid first optical surface of said first gain medium substantiallyconforms to and contacts said first optical surface of said first heatconductor at a second optical output condition, wherein heat conductioncan occur from said first gain medium to said first heat conductorthrough an interface formed by said first optical surface of said firstheat conductor and said first optical surface of said first gain mediumat said second optical output condition; wherein said first opticalsurface of said first gain medium is concave toward said first opticalsurface of said first heat conductor at said first optical outputcondition.
 2. The system of claim 1, further comprising a means forpumping operable to produce pump energy, said means for pumpingconfigured and arranged to transmit said pump energy to said first gainmedium.
 3. The system of claim 1, wherein said resonator is a stableresonator.
 4. The system of claim 1, wherein said resonator is anunstable resonator.
 5. The system of claim 2, wherein said means forpumping comprise one or more diode bars.
 6. The system of claim 1,wherein said first heat conductor comprises diamond.
 7. The system ofclaim 6, wherein said diamond is single crystal diamond.
 8. The systemof claim 2, wherein said means for pumping is configured and arrangedsuch that pump light is incident on said first transparent heatconductor at the Brewster angle between the first heat conductor and anintermediary optical medium located between said means for pumping andsaid first heat conductor.
 9. The system of claim 8, wherein saidintermediary optical medium is air.
 10. The system of claim 8, whereinsaid intermediary optical medium is water.
 11. The system of claim 2,further comprising a beam combiner disposed on said optical path in saidresonator to receive said pump energy from said means for pumping,wherein said beam combiner is configured and arranged to transmit saidpump energy along said optical path.
 12. The system of claim 11, whereinsaid beam combiner comprises one or more Brewster prisms.
 13. The systemof claim 4, wherein said first mirror comprises a graded reflectivitymirror, wherein said graded reflectivity mirror includes one or moreoptical coating with graded reflectivity profiles disposed on a firstoptical surface of said first mirror supports a single transverse modealong said optical axis.
 14. The system of claim 13, wherein said gradedreflectivity mirror includes a Gaussian reflectivity profile.
 15. Thesystem of claim 1, further comprising one or more dichroic coatingsdisposed on said first and second mirrors within said resonator, whereinsaid dichroic coatings transmit said pump wavelength and have a desiredreflectivity for a wavelength generated by said first gain medium.
 16. Amethod of producing light using a self-correcting gain medium inconjunction with a transparent heat conductor, said method comprisingthe steps of: placing a first gain medium having first and secondoptical surfaces, said first optical surface having a thermallyself-correcting shape, adjacent a heat conductor having first and secondoptical surfaces and being transparent at one or more desired opticalwavelengths, wherein said thermally self-correcting shape is dissimilarto said first optical surface of said heat conductor at an initial levelof optical output of said first gain medium; adding pump energy to saidfirst gain medium; adding thermal energy to said first gain medium;producing a desired optical wavelength with said first gain medium;conforming said first optical surface of said first gain medium to saidfirst optical surface of said heat conductor at a desired level ofoptical output of said first gain medium; removing heat axially fromsaid first gain medium through said first optical surface of said heatconductor; and removing heat radially from said heat conductor; whereinplacing the first gain medium having first and second optical surfacesadjacent the heat conductor having first and second optical surfacescomprises placing the first gain medium having first and second opticalsurfaces, said first optical surface having the thermallyself-correcting shape, adjacent the heat conductor having first andsecond optical surfaces and being transparent at one or more desiredoptical wavelengths, wherein said thermally self-correcting shape of thefirst optical surface defines a concave cavity relative to said firstoptical surface of said heat conductor at the initial level of opticaloutput of said first gain medium.
 17. The method of claim 16, whereinsaid step of adding thermal energy to said first gain medium comprises astep selected from the group consisting of amplifying fluorescence,amplifying a signal wave, amplifying an idler wave, and amplifying apump wave.
 18. The system of claim 1, wherein said first optical surfaceof said first gain medium has a substantially Gaussian orHermite-Gaussian profile relative to said optical axis at said firstoptical output condition.
 19. The method of claim 16, wherein saidthermally self-correcting shape of the first optical surface defines asubstantially Gaussian or Hermite-Gaussian profile relative to saidoptical axis at the initial level of optical output of said first gainmedium.
 20. The system of claim 1, wherein when said first opticalsurface of said first gain medium substantially conforms to and contactssaid first optical surface of said first heat conductor at a secondoptical output condition, said concave portion of said first opticalsurface thermally expands such that the first gain medium substantiallyconforms to and contacts said first optical surface of said first heatconductor at the second optical output condition.
 21. The method ofclaim 16, wherein conforming said first optical surface of said firstgain medium to said first optical surface of said heat conductor at adesired level of optical output of said first gain medium comprisesthermally expanding said concave portion of said first optical surfacesuch that the first gain medium substantially conforms to and contactssaid first optical surface of said first heat conductor.